A magnetic tunneling junction (MTJ) is formed by interposing a thin insulating layer between a pair of magnetic layers. When a voltage is applied between the two magnetic layers, electrons in one of the magnetic layers enter the other layer by passing through the insulating layer by quantum mechanical tunneling. The electrical resistance of the MTJ varies according to the direction of magnetization of the two magnetic layers. In particular, the electrical resistance of the MTJ has a minimum value when the directions of magnetization of the two magnetic layers are in parallel with each other, and has a maximum value when the directions of magnetization of the two magnetic layers are in an anti-parallel relationship. If the direction of magnetization of one of the magnetic layers is changed by an applied magnetic field (external magnetic field) while the direction of magnetization of the other magnetic layer remains fixed, the electrical resistance value of the MTJ changes according to the direction of the applied magnetic field. This changing of the direction of the magnetization is referred to as “switching.” Information can be stored in each MTJ and read out (i.e., reproduced) from the MTJ by sensing the tunneling current value.
One useful application of a MTJ is in a type of nonvolatile memory device known as a magnetic random access memory (MRAM) device. Each memory cell of the MRAM device incorporates a MTJ, and a plurality of memory cells are arranged in an addressable array. Magnetic tunneling junctions that have been employed in MRAM devices utilize magnetic elements having solid elliptical or modified rectangular shapes (e.g., hexagon). Such shapes exhibit a linear magnetization mode that produces several problems when they are scaled down to nanometer-sized elements. For example, edge magnetic domain, 360° domain walls, and localized vortices will occur leading to multiple magnetic domains. Such multiple magnetic domains cause unrepeatable switching and a non-uniform magnetization configuration of the magnetic element. Another significant issue with conventional magnetic element geometries, especially generally rectangular shapes with modified end portions, is the existence of a wide switching field distribution. Consequently, a large switching error occurs on individual memory cells.
In order to address the deleterious effects of the above-described magnet element shapes, a circular or an asymmetric circular magnet element has been proposed. However, for solid disc-magnetic elements having an asymmetric circular shape the vortex core interrupts the stability of the magnetic configuration in sub-micron-sized elements. More specifically, the vortex core is positioned on the center of a symmetrical disc, but in an asymmetrical design the location of the vortex core is not well defined. Consequently, the vortex core is unstable resulting in a high switching field and a wide switching field distribution. A ring-shaped magnetic element has also been proposed (see Prinz, U.S. Pat. No. 5,969,978). A ring-shaped magnetic element includes an isolated cavity or void that is difficult to accurately pattern with existing fabrication processes.